The present invention relates to analytical devices for detecting and characterizing minute particles and macromolecules suspended or dissolved in liquid samples, and more particularly to a means for generating droplets of the liquid samples, of a size and uniformity to enhance the effectiveness of such analytical devices.
The ability to analyze liquid solutions is becoming increasingly important in a wide variety of fields including medicine, pharmaceuticals, production of polymers, paints and dyes, environmental science and genetics. A variety of techniques including atomic absorption spectrometry, atomic emission spectrometry, inductive coupled plasma, light scattering, and mass spectrometry, are used to detect, characterize and determine concentrations of solutes, suspensions and residue in liquid solutions. In connection with these techniques, it is highly preferred to convert the liquid sample into aerosol form, typically by using a nebulizer.
While various types of nebulizers are known, including ultrasonic, pneumatic, frit and thermospray, an electrospray nebulizer is preferred in many applications due to its ability to generate small and uniform droplets, and in its relatively high efficiency, in terms of sample droplets delivered to a detector as compared to the sample uptake rate.
In the electrospray nebulizer, electrically conductive liquid is supplied at a controlled rate to a capillary tube. A voltage differential between the capillary tube and a surrounding chamber wall creates an electrostatic field that induces a surface charge in liquid emerging from the tube. Electrostatic or "Coulomb" forces disperse the liquid into a fine spray of charged droplets. To produce the spray, each droplet is charged near the Rayleigh limit (at which point electrostatic repulsion overcomes surface tension).
When analyzing macromolecules, colloids or other small particles of interest, the particles are dispersed in a liquid, the liquid is sprayed in small droplets, then the droplets are dried, leaving the particles in aerosol form. An exemplary use of an electrospray nebulizer is disclosed in U.S. patent application Ser. No. 07/564,004, filed Jun. 28, 1990 and assigned to the assignee of this application. An apparatus for measuring concentrations of macromolecules and colloids in a liquid sample, uses an electrospray atomizer to receive a liquid analyte, after separation by a liquid chromatography system. Within the atomizer, an electrical field charges the liquid emerging at the tip of a needle, whereby the liquid is dispersed into a fine spray of charged droplets. As solvent evaporates from each droplet, charge density on the droplet surface increases until the Rayleigh limit is reached. The resulting instability causes the droplet to disintegrate into smaller droplets. The aerosol output of the electrospray atomizer is provided to a condensation nucleus counter, either directly or through diffusion screens that filter smaller particle sizes.
Even "pure" liquids contain some non-volatile material. Accordingly, each droplet contains a proportion of a residue, and further may contain one of the particles under study. The particle concentration in the liquid sample, and the volume of the droplets as initially formed, are kept to a minimum to avoid production of "clusters" (droplets containing two or more of the particles under study).
The size of residue particles depends upon residue concentration and initial droplet size. For example, a one part per million impurity level results in a residue particle of about one percent of the diameter of the original droplet (assuming the dried material and the liquid have approximately the same density). Thus, a nebulizer producing droplets 10 micrometers in diameter would produce residue particles having a diameter of about 100 nanometers.
Such residue particles are inconsequential, so long as particles under study are relatively large. However, residue particles cause substantial interference or artifacts that interfere with detecting and characterizing smaller particles. In many of the abovementioned fields, there is a strong interest in studying particles as small as three nanometers in diameter, e.g. macromolecules and colloids such as synthetic polymers, proteins, viruses and particles of concern in connection with maintaining semiconductor production facilities. To reduce residue artifacts, it is necessary either to reduce the non-volatile impurity concentration, or to generate smaller droplets.
Given the difficulty in producing and handling ultra pure liquids, generating smaller droplets appears to be the logical solution. However, the Coulomb forces employed to generate droplets also cause droplet disintegration shortly after formation. More particularly, as liquid evaporates from the droplets, surface charge density on the droplets increases until the Rayleigh limit is reached, at which point the Coulomb repulsive force becomes the same order as cohesive forces such as surface tension. The resulting instability causes the original droplet, sometimes referred to as the parent or primary droplet, to disintegrate into smaller droplets. The primary droplet appears to eject several small droplets, removing a substantial proportion of the total charge. The parent droplets and fragments continue to evaporate and can experience further fragmentations. The resulting distribution of droplet sizes is broad, i.e. non-uniform.
Therefore, it is an object of the present invention to provide a device for generating uniform droplets having diameters of less than one micrometer.
Another object of the invention is to provide a system for detecting macromolecules, colloids and other particles of interest having diameters in the range of 3-1,000 nanometers, substantially without interference from residue artifacts.
A further object is to provide a source of uniform aerosols for testing particle detection and classification devices within precisely defined ranges of particle diameters.
Yet another object is to provide a means for preparing uniform droplets sufficiently small to isolate single individual biological molecules, e.g. nucleic acids, proteins, and carbohydrates, for subsequent observation.